DC energy stores can be implemented from many different technologies such as batteries (e.g. lithium and sodium sulphur type), capacitors including supercapactitors and ultracapacitors, and flow cells (e.g. vanadium redox flow cells). Energy can be stored in the dc energy store and released when it is needed. This ability to store and release energy is useful in a number of different applications including storing excess wind energy and releasing that energy to an ac supply network or power grid at a later time when less wind energy is available. A different application is to store energy in case an electrical generator stops operating. The energy stored in the dc energy store can then be used to keep essential systems and services operating until a standby or back-up generator is started and brought into operation. In some circumstances it can also be helpful to store energy slowly and allow the energy stored in the dc energy store to be released very quickly for applications that need high power pulses of energy like the linear electrical motors that are used to start roller coasters.
The dc energy store will typically have auxiliary systems such as pumps, fans, control units and other devices that are connected to the ac supply network. If the ac supply network is interrupted or experiences voltage or frequency transients then the auxiliary systems will not receive power and this can cause problems with the practical operation of the dc energy store. For example, if the fans that are used to cool the dc energy store stop working then this can result in unacceptable temperature increases. Certain types of dc energy store also require pumps to continue running at their optimum efficiency.